The unideterminant compound L-tyrosine-p-azophenyltrimethylammonium (tyr(TMA)) can induce cellular immunity in guinea pigs with no detectable antibody produced to this immunogen. If a second identical tyr(TMA) determinant is covalently linked via a 1,6-diaminohexane spacer molecule to tyr(TMA), this symmetrical bifunctional compound, (tyr(TMA)-NH-(CH2)3)2, regularly induces antityr(TMA) antibody. The well-defined antigen offers the unique opportunity in that both T and B cells react and cooperate to identical linked determinants to produce antibodies. It is proposed to compare the antigen recognition unit(s) on tyr(TMA) specific T and B cells by using idiotypic probes. Idiotypic antisera made against anti-tyr(TMA) antibody elicited by the symmetrical bifunctional tyr(TMA) antigen will be used to examine shared V-region idiotypes. Cross-reactivity of idiotypes expressed by the antigen receptors will be determined by the ability of the anti-idiotypic serum to block antigen binding, MIF production, and lymphocyte proliferation in T-cell populations. If the idiotypic antisera effectively blocks both antigen binding and T-cell activation, this would provide compelling evidence for shared V-region receptors on both T and B cells. If indeed V-regions play a role in antigen recognition, then it is theoretically possible to immunoregulate via idiotypic antisera in a very specific fashion. This is an important concept for basic immunology providing we also understand the nature of the specific antigen(s) in question.